1. Field of the Invention
The present invention relates to a heat treatment method for heating a thin plate-like precision electronic substrate such as a semiconductor wafer and a glass substrate for a liquid crystal display device (hereinafter referred to simply as a “substrate”) by irradiating the substrate with a flash of light.
2. Description of the Background Art
In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ions of impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the subsequent annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might constitute a hindrance to good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate a surface of a semiconductor wafer with a flash of light, thereby raising the temperature of only the surface of the semiconductor wafer doped with impurities in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with a flash of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that flash irradiation, that is, the irradiation of a semiconductor wafer with a flash of light in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
A heat treatment apparatus which employs such xenon flash lamps is disclosed in U.S. Pat. No. 6,998,580 in which the flash lamps are disposed on the front surface side of a semiconductor wafer and a thermal diffuser and a hot plate are disposed on the back surface side thereof so that a desired heat treatment is performed using a combination of these components. In the heat treatment apparatus disclosed in U.S. Pat. No. 6,998,580, a semiconductor wafer is placed on the thermal diffuser, and is preheated to a certain degree of temperature by the hot plate. Thereafter, the temperature of the semiconductor wafer is raised to a desired treatment temperature by the irradiation with flashes of light from the flash lamps.
However, a heat treatment apparatus employing such xenon flash lamps as disclosed in U.S. Pat. No. 6,998,580, which momentarily irradiates the front surface of a semiconductor wafer with a flash of light having ultrahigh energy, raises the temperature of the front surface of the semiconductor wafer rapidly for a very short period of time to cause abrupt thermal expansion of the front surface of the semiconductor wafer, so that the semiconductor wafer tends to become deformed. However, the thermal diffuser on which the semiconductor wafer is placed prevents such deformation, so that reaction stresses are exerted on the semiconductor wafer. As a result, there is a danger that wafer cracking occurs with a high probability.